1. Field of the Invention
The invention relates to crashworthy seats for use in aircraft, particular helicopters.
2. Description of the Prior Art
The recent sharp increase in military and civilian use of helicopters has induced a concurrent need for improved means for protecting the safety of occupants, particularly in a military context, under crash conditions. Substantial research has been accomplished on the nature of helicopter crashes, and particular attention has focused upon the subject of defining a single crash pulse most accurately representing the most probable type of crash for helicopter type aircraft. A particular pulse shape has been discerned and has been accepted by virtually all military agencies in the United States. Thus, it is this shape which must be met in providing safety for occupants of helicopter seats, under crash conditions.
Besides meeting the requirements of the most probable crash pulse referred to above, further requirements of an effective crashworthy helicopter seat include safety for users having a range of body weight. Studies have indicated that the maximum tolerable deceleration along the human spine for appreciable time intervals, i.e., 0.006 second, is approximately 23 g's. Under a vertorial analysis of the accepted most probable crash pulse, the main acceleration vector is that identified along the spine of the occupant. Such a crash pulse at an impact velocity of 50 feet per second will cause a peak deceleration of approximately 48 g's. This is substantially higher than the maximum tolerable deceleration. Since force levels in energy absorber elements are fixed at prescribed levels, "g" values experienced will increase with decrease of weight of the occupant of the seat. Because operating distance for energy absorber elements in aircraft is fixed, force levels in existing energy absorbers are determined by the available operating distance in the aircraft for attenuating for the largest occupant weight without permitting the seat to contact the floor of the aircraft with appreciable velocity. This has the effect of penalizing lighter weight seat occupants subjecting them to possible injury or death in crashes from which persons of greater body weight would emerge relatively uninjured.
In U.S. Pat. Nos. 3,369,634 and 4,150,805, both to Applicant, energy absorber systems were disclosed and claimed capable of use with crashworthy helicopter seats, for the purpose of protecting occupants thereof from injury resulting from the extreme accelerations experienced in aircraft crashes. In copending application Ser. No. 192,958 is disclosed and claimed a system of energy absorbers with parameters adaptable to optimizing safety of helicopter seat occupants of particular weight percentiles. In particular applications, it has been found useful to provide an energy absorbing system which is adjustable, in place, to optimize operational characteristics for occupants in differing weight percentiles.